1. Field of Invention
The present invention relates to a protective layer for electrophotographic photoreceptors, and process cartridges and image forming apparatuses containing photoreceptors having protective layers.
2. Description of Related Art
In xerography, or electrophotographic printing/copying, a charge-retentive device called a photoreceptor is electrostatically charged. For optimal image production, the photoreceptor should be uniformly charged across its entire surface. The photoreceptor is then exposed to a light pattern of an input image to selectively discharge the surface of the photoreceptor in accordance with the image. The resulting pattern of charged and discharged areas on the photoreceptor forms an electrostatic charge pattern (i.e., a latent image) conforming to the input image. The latent image is developed by contacting it with finely divided electrostatically attractable powder called toner. Toner is held on the image areas by electrostatic force. The toner image may then be transferred to a substrate or support member, and the image is then affixed to the substrate or support member by a fusing process to form a permanent image thereon. After transfer, excess toner left on the photoreceptor is cleaned from its surface, and residual charge is erased from the photoreceptor.
Electrophotographic photoreceptors can be provided in a number of forms. For example, the photoreceptors can be a homogeneous layer of a single material, such as vitreous selenium, or it can be a composite layer containing a photoconductive layer and another material. In addition, the photoreceptor can be layered. Current layered photoreceptors generally have at least a flexible substrate support layer and two active layers. These active layers generally include a charge generating layer containing a light absorbing material, and a charge transport layer containing electron donor molecules. These layers can be in any order, and sometimes can be combined in a single or a mixed layer. The flexible substrate support layer can be formed of a conductive material. Alternatively, a conductive layer can be formed on top of a nonconductive flexible substrate support layer.
An electrostatographic photoreceptor can be in a rigid drum configuration or in a flexible belt configuration that can be either a seamless or a seamed belt. Typical electrophotographic photoreceptor drums comprise a charge transport layer and a charge generating layer coated over a rigid conducting substrate support drum. However, for flexible electrophotographic photoreceptor belts, the charge transport layer and charge generating layer are coated on top of a flexible substrate support layer. To ensure that the photoreceptor belts exhibit sufficient flatness, an anticurl backing layer can be coated onto the back side of the flexible substrate support layer to counteract upward curling and ensure photoreceptor flatness.
In many modern electrophotographic imaging systems the flexible photoreceptor belts are repeatedly cycled to achieve high speed imaging. As a result of this repetitive cycling, the outermost layer of the photoreceptor experiences a high degree of frictional contact with other machine subsystem components used to clean and/or prepare the photoreceptor for imaging during each cycle. When repeatedly subjected to cyclic mechanical interactions against the machine subsystem components, photoreceptor belts can experience severe frictional wear at the outermost organic photoreceptor layer surface that can greatly reduce the useful life of the photoreceptor. Ultimately, the resulting wear impairs photoreceptor performance and thus image quality.
In order to mitigate erosion of the top outermost layer during these processes, the outermost layer can be coated with a thin protective layer, such as the overcoat disclosed in U.S. Patent Application Publication No. US 2004/0086794 A1, incorporated herein by reference in its entirety.